In a cable TV system, a distribution system is used to carry a cable TV signal from an origination point, referred to as a "headend", to a television at the subscriber's premises. An exemplary cable TV distribution system is depicted at 10 in FIG. 1. A headend of the cable system 10 is indicated at 12 and typical subscribers' premises are indicated at 14. The headend 12 combines separate information carrying signals into an RF cable TV signal. A television set (not shown) at the subscribers' premises 14 selects one of the information carrying signals from the cable TV signal, decodes this signal, and displays the decoded information to a subscriber.
As shown in FIG. 1, the information carrying signals that are combined to form the cable TV signal may be supplied to the headend 12 by such sources as a satellite receiver 16, a point-to-point microwave receiver 18, a video tape player 20, or a studio 22.
A system for distributing the cable TV signal from the headend 12 to the subscribers' premises 14 is indicated at 24 in the drawing. This cable system 24 basically comprises a trunking system 26 and a distribution system 28.
The trunking system 26 is designed to conduct the cable TV signal from the headend 12 to the distribution portion 28. Three common methods of implementing such a trunking portion 26 are a coaxial cable trunking system, a point-to-point microwave trunking system, or a fiber optic cable trunking system. The details of the these trunking systems are not per se part of the invention and will not be discussed in further detail below.
The cable TV signal is carried over coaxial cables once it arrives at the distribution system 28. The distribution system 28 generally comprises: (a) a series of main coaxial cables 30; (b) one or more amplifiers 32 referred to as line extenders located in the series of main coaxial cables 30; (c) one or more power supplies 34; (d) a power inserter 36 for each of the power supplies 34; (e) one or more taps 38; and (f) low capacity coaxial drop cable 40 extending between each tap 38 and the subscribers' premises 14 associated therewith.
The line extenders 32 are employed to maintain signal strength as the cable TV signal is distributed over the main coaxial cables 30. These line extenders 32 obtain power from an AC power signal generated by the power supplies 34 and introduced into the main coaxial cables 30 through the power inserters 36. The main coaxial cables 30 branch off to feed the taps 38.
The problem addressed by the present invention is caused by an interaction between the AC power signal generated by the power supplies 34 and the components within the taps 38. Accordingly, to understand the nature of this interference problem, the details of construction and operation of currently available power supplies 34 and taps 38 will now be described in further detail.
The power supplies 34 are designed to operate in two modes of operation: (a) a line mode in which power is supplied through utility power lines; and (b) a standby mode in which power is supplied by a battery or series of batteries. A simplified block diagram of an exemplary power supply 34 is depicted in FIG. 2. Power supplies such as that shown in FIG. 2 are well-known and currently available on the market from several different sources.
The power supply 34 basically comprises an AC module 42, an inverter module 44, a battery 45, a connection 46 to line voltage, a line sensing circuit 48, and a transfer relay 50. The power supply 34 operates in the following manner. During normal operation, the AC module 42 generates a line AC power signal from the line voltage; in such normal operation, the transfer relay 50 is arranged to allow this line AC power signal to pass to an output terminal 51. When the line sensing circuit 48 determines that a fault exists in the line voltage, the line sensing circuit 48 sends a signal to the inverter module 44 to begin generating a standby AC power signal. The sensing circuit 48 also sends a signal to the transfer relay 50 to connect the inverter module 44, rather than the AC module 44, to the output terminal 51.
The operation and construction of the AC module 42, which basically comprises a ferroresonant transformer and an output capacitor, is well-known, is not directly relevant to the present invention, and thus will not be discussed in further detail.
A typical inverter circuit comprising the elements of the inverter module 44 and the battery 45 of the known power supply 34 is shown at 52 in FIG. 3. Basically, this inverter circuit 52 comprises a frequency source 54, an inverting element 56, first and second drive circuits 58 an 60, first and second switching transistors 62 and 64, a latching element 66, and a linear transformer 68. The transformer 68 has first and second windings 70 and 72. A positive terminal 74 of the battery 45 is connected to a center tap 76 of the first winding 70; a negative terminal 78 of the battery 45 is connected to ground.
The first and second switching transistors 62 and 64 are connected at their bases to the first and second drive circuits 58 and 60, respectively. The emitters of these transistors 62 and 64 are connected to ground, while the collectors thereof are connected to opposite ends of the first winding 70.
The ends of the second winding 72 are connected to output terminals 78 and 80.
This inverter circuit 52 operates in the following basic manner. The frequency source generates a 60 Hz square wave. This square wave is applied to the first drive circuit 58 and the inverting element 56. The inverting element 56 generates an inverted square wave that is applied to the second drive circuit 60. The latch element 66, in response to a signal generated by the line sensing circuit 48, allows the square wave and its inverted counterpart to reach the first and second drive circuits 58 and 60 when the line sensing circuit 48 senses a fault in the output of the AC module 42.
In response to the square waves, the first and second drive circuits provide an appropriate voltage to the bases of the switching transistors 62 and 64 to turn these transistors on when the square waves are high. Further, because the square wave inputs to the first and second drive circuits are inverted from each other, the switching transistor 62 is "ON" when the switching transistor 64 is "OFF", and vice versa.
When either of the transistors 62 and 64 is "ON", current flows from positive terminal 74 of the battery 45, through the transformer center tap 76, out the appropriate end of the transformer first winding 70, through the "ON" transistor, and to ground (battery negative terminal 78).
The transformer 68 is a linear transformer. Therefore, for a battery 45 having a DC voltage V.sub.B, the above-described system generates at the output terminals 80 and 82 the standby AC power signal. The standby AC power signal is a square wave signal having a peak voltage of approximately V.sub.B.
This square wave standby AC power signal has heretofore been considered desirable for at least two reasons. First, the inverter module 44 operates most efficiently when generating such a square wave. Specifically, the switching transistors 62 and 64 operate most efficiently when they are either "ON" or "OFF". When they are "ON", they act like a short circuit, and thus very little energy is dissipated therein. When they are "OFF", they act like an open circuit, allowing substantially no current to pass therethrough and thus consuming very little or no power. The square wave AC power signal requires these transistors to be switched quickly between "ON" or the "OFF", thus spending most of the time in their most efficient states.
The second reason such a square wave AC power signal is considered desirable is because, as is well-known in the art, the line extenders 32 can efficiently convert such a square wave signal into a DC power signal.
Referring now to FIG. 4, shown therein is a schematic of a typical tap 38. A typical tap 64 basically comprises a power passing choke 84, a coupling transformer 86, and first and second RF coupling capacitors 88 and 90. The first capacitor 88 and coupling transformer 86 filter out the AC power signal generated upstream by the power supplies 34 and reduce the voltage of the RF cable TV signal to a level appropriate for the subscriber's television. The drop cables 40 extend from the output of the coupling transformer 86 to the subscriber's premise 14. The choke 84 and second RF coupling capacitor 90 allow the RF cable TV signal and the AC power signal to pass through the tap 38.
A primary function of the tap 38 is to allow the cable TV signal to be dropped to a number of subscriber's premises from a single point on the distribution cable 30. Other important functions of these taps 64 are to: (a) reduce the voltage level of the signals entering the subscribers' premises 14; and (b) isolate the low capacity coaxial drop cables 40 feeding the subscribers' residences 14 from the distribution coaxial cables 30.
The AC power signal generated by the inverter circuit 52 described above has been found to cause the taps 38 to generate an interference signal that interferes with the cable TV signal entering the subscriber's premises through the drop cable 40.
The interference signal has been attributed at least in part to the inverter circuit 52 because this interference has been found to be more likely to occur when the power supply is in standby mode; this interference problem is much less likely to occur when the AC power signal is being generated by the AC module 42 connected to the line voltage.
Also, it has been discovered that this interference problem is much mores severe: (a) for a given range of values chosen for the capacitors employed in the taps; and (b) when large numbers of taps are attached in series to a single power supply. Accordingly, the taps are also believed to be responsible for this interference problem.
An example of lines in the television picture caused by the above-described interference signal is depicted in FIG. 5.
Several steps may be taken to alleviate to some extent the interference problem solved by the present invention.
As one option, the capacitance values of the capacitors within the taps may be reduced. Previously, in an attempt to increase the bandwidth of the signal that may pass through the taps, at least one tap manufacturer has increased the values of the capacitors within the taps. The taps with such increased capacitor values are more susceptible to the interference problem described above. Accordingly, the problem may be alleviated to some extent by providing capacitors in the taps with smaller capacitance values.
However, it is generally desirable, in order to allow increased bandwidth of the signal that may be passed through the taps, not to rely on these smaller capacitance values to solve the interference problem addressed by the present invention. Further, given the number of taps currently installed, it is not economical change the taps or the capacitors within these taps at this time.
As a second option, the number of taps in the line downstream from each power supply may be reduced. Placing a large number of taps in series downstream of a given power supply increases the voltage spikes occurring towards the end of the line. This is because the signal is passed through the differentiating circuits formed in a number of successive taps, with the peak of the voltage spikes being increased by each tap. Theoretically, the present problem may thus be alleviated by decreasing the number of taps connected in the line downstream of each power supply by increasing the number of power supplies.
However, it is not practical in a cable TV system to decrease the number of taps in this manner because to do so would require the purchase and installation of a large number of relatively costly power supplies.